1. Field of the Invention
The present invention relates to an image reading apparatus, an image forming apparatus, and a method of image reading, which use white light produced in use of light emitted from LEDs as white light to be irradiated to documents, for example.
2. Description of the Related Art
As a light source for scanning a document in an image reading apparatus, such as a scanner, provided in an image forming apparatus, a light source that uses a white LED is available, with such a white LED being constituted by a blue LED that emits blue light in response to a drive current and an yttrium-aluminum-garnet (YAG) phosphor that converts the blue light into yellow light, and white light is emitted by the phosphor emitting fluorescence with the blue light from the blue LED.
The emission wavelength for the white LED has a specific profile (spectral intensity profile) as shown by solid line Wn in FIG. 2 for example. More specifically, the characteristic curve of the profile indicates intensity peaks at the wavelengths at about 460 nm and about 580 nm, has a narrow wavelength distribution width in the blue wavelength range (400 nm to 500 nm) and a wide wavelength distribution width in the yellow wavelength range (550 nm to 600 nm).
When light is illuminated to an object, it is partly reflected and the remaining fraction is absorbed or transmitted. The color tones of the object are determined by the wavelengths of light that are partly reflected. For example, when an object reflects red wavelengths more than others, this object appears red. An object that reflects red wavelengths appears white when it also reflects other wavelengths as well.
Some images sensors such as CCDs are known to utilize this principle. When white light emitted from the white LEDs is illuminated to the document, some fraction is reflected therefrom as the reflected light. The image sensor receives the reflected light and photoelectrically converts it into image data indicating density values of each of RGB with predetermined RGB spectral sensitivity (spectral sensitivity characteristic) according to wavelength of the received light. The image sensor thus produces the image data corresponding to the scanned document.
As shown in FIG. 4 for example, the image sensor converts the spectrum of the received light having wavelengths in the region of about 400 nm to about 700 nm, into image data indicating density value of red (R) based on the sensitivity shown as dotted line in the figure. Likewise, the image sensor converts the spectrum of the received light having wavelengths in the region about 400 nm to about 700 nm, into image data indicating density value of green (G) based on the sensitivity shown as solid line in the figure. It also converts the spectrum of the received light having wavelengths in the region about 400 nm to about 580 nm, into image data indicating density value of blue (B) based on the sensitivity shown as dash-dotted line in the figure. The higher the sensitivity for each of RGB colors, the higher the density values of each of RGB when the light corresponding to the wavelengths in that region is received.
The image sensor does not always receive the light with the constant intensity for all the wavelengths. Instead, it may receive light with different intensities for different wavelength ranges, as shown in, for example, FIG. 2. In this case, the image sensor increases density values of each of RGB when received the light of the wavelength range, according to increase the intensity of light different from depending on the wavelength range of the received light.
However, following problems arise when the white light emitted from the white LED having the spectral intensity profile as shown in FIG. 2 is irradiated to a document having blue tone, and the reflected light reflected from the document is converted into image data using an image sensor having the spectral sensitivity characteristic as shown in FIG. 4. The reflected light reflected from the document has a range of blue wavelengths that is narrower than a range for the natural blue tone of the document because the white light to be irradiated to the document has a narrow range of blue wavelengths. This may result in an image data having a low density value of blue (B) when converted by an image sensor.
This problem does not occur when the white light emitted from the white LED having the spectral intensity profile as shown in FIG. 2 is irradiated to a document having yellow tone, and the reflected light reflected from the document is converted into image data using an image sensor having the spectral sensitivity characteristic as shown in FIG. 4. In this case, the white light to be irradiated to the document has a wider range of yellow wavelengths. The yellow wavelengths of the reflected light reflected from the document encompass all wavelengths for the natural yellow tone of the document. The image sensor converts the light into an image data having high density values of red (R) and green (G), that is to say, as a mixed color of red (R) and green (G) that appear yellow to our eyes.
As described above, there is a problem that some color tones cannot be precisely reproduced as image data due to wavelength distribution of the light irradiated from a light source to a document.